The invention relates to an apparatus for producing shock waves for technical, preferably medical, applications, in particular for lithotripsy or pain therapy, in which mechanical waves with high energy density are produced through the use of pressure pulsations.
Intense sound waves or shock waves with working pressures in a range of several 107 Pa up to 108 Pa are used for various applications. One example is lithotripsy in medicine, in which focused pressure waves generated outside the body are used to generate a strong shock wave at the location of gallstones or kidney stones, which is so strong that the stone disintegrates into small fragments which can leave the body in the natural way without surgical intervention. Typically, several hundred to several thousand shock wave applications, i.e. individual pulses, are required to ensure sufficiently high fragmentation of the stone.
In order to generate the latter shock waves, there is a need for a shock wave generator which generates a sound wave that is already focused or can be focused by lenses, in particular acoustic lenses, and the focus of which must be at the location of the stone to be destroyed. The focal length of the acoustic configuration should be small, i.e. in the range of some tens of centimeters, in order to limit the energy density at the surface of the patient""s body, i.e. to  less than 1 J/cm2. That permits the pain caused by the passage of the sound to be controlled by local anesthetics.
The pulse repetition rate should be about 1 to 5 per second for an acceptable treatment time. The life of the shock wave generator should be as long as possible, i.e. several million pulses, to allow a relatively large number of patients to be treated without the need for servicing or repair work. The properties of the shock wave generator, in particular shock wave energy, pulse duration, position of the focus etc., should only change slightly, if at all, during its entire life in order to permit constant, reproducible results. The shock waves should be generated in water or in liquids with acoustic properties comparable to those of water to ensure efficient propagation and transmission of the sound into the body of the patient through a suitable acoustic impedance between the shock wave generator and the body. The focus diameter of the focused shock wave at the location of the stone (xcx9ccm) should be comparable with the diameter of the stone to ensure an efficient interaction between the shock wave and the stone. Typical wavelengths for the shock wave are in a range from 1 to 10 mm, corresponding to pulse durations of, typically, xcx9c1 xcexcs. Quality requirements at the wave front in the shock wave generator to enable the required focusing ability to be achieved are correspondingly high.
The requirements are similar in other technical applications, e.g. in recycling through the use of shock waves, in cleaning surfaces through the use of shock waves, in mining, breaking up rock without the use of chemical explosives, for example, in geology and in oceanography, for sonar applications for example. In some of those applications, considerably higher and, in some cases, more variable, pulse energies are required than in lithotripsy. Therefore, a virtually arbitrarily scaleable shock wave generator principle would be very useful for many applications.
Apart from using chemical explosives, the following three principles are the only ones used heretofore for generating shock waves. According to those principles, electrical energy is converted to acoustic energy in the form of intense shock waves:
the electrohydraulic principle involving the generation of a spherically expanding pressure wave through the use of an underwater spark and, if required, focusing with ellipsoidal reflectors, such as is described in Rev.Sc. Instrument 65 (1994), pp. 2356-2363 and Biomed. Tech. 22 (1977), p. 164 ff;
the piezoelectric principle involving the generation of a pressure wave by using pulsed piezoelectric sound transducers, as described, for example, in German Published, Non-Prosecuted Patent Application DE 33 19 871 A1, corresponding to U.S. Pat. No. 4,858,597; and
the electromagnetic principle involving the generation of a pressure wave through the use of an electromagnetically driven diaphragm, which is described in detail in Appl. Phys. Lett. 64 (1994), pp. 2596-2598 and Acustica 14 (1964), p. 187.
Particularly in the case of the principle first mentioned above, the main disadvantages are short service life, poor reproducibility and limited scaleability of the shock wave transducers, and short service life, e.g. just a few thousand pulses due to electrode erosion and an associated fluctuation in the position of the focus, which in particular present problems. Piezoelectric transducers likewise have a very limited mechanical service life at the amplitudes which are required in that case. At present, electromagnetic sound transducers have the longest service lives, typically xcx9c1 million pulses. However, for reasons connected with their ability to withstand electrical and mechanical loading, they can only be scaled to a limited extent. Extending the service life to several million pulses would be advantageous, as would wider scaleability of the shock wave energy and pulse shape.
In order to implement the electrohydraulic principle, German Patent DE 0 911 222 C has disclosed a sound transmitter in which the sound pressure is generated when a current passes through shock-like vaporizations brought about in narrowly defined liquid filaments. German Published, Prosecuted Patent Application DE 10 76 413 B has already disclosed a sound generating method in which a field line contraction on a wire or at an end of a wire or at the constriction caused by a flexible insulating body is used to achieve a high field density and consequently a high power density in the immediate vicinity of the wire. However, that only allows small volumes in the immediate vicinity of the wire or at the constriction to be used. As a result, on one hand the majority of the energy is converted at low energy density in large volumes, thereby drastically reducing the energy content of the pressure wave and efficiency and, on the other hand the achievable energy is very small due to the small volume. In practice, connecting a large number of such channels in parallel has the effect that, due to slight differences between the channels, a single channel is preferred and it is then heated up to a greater extent than the others. The earlier and higher current flow resulting from the higher temperature generally leads to a flashover of high current intensity and, because of the non-linearity of the processes leading to the flashover, the principle can thus only be used at safe power densities well below the breakdown strength of the electrolyte. That imposes a severe limit both on the amplitude and on the efficiency of such a pulsed sound source. Even slight differences in the channels lead to significant fluctuations in the associated pressure amplitudes. As a result, homogeneous wave fronts can only be produced to a limited extent with such a system.
Finally, U.S. Pat. No. 5,105,801 has disclosed a configuration in which two discharge electrodes are aligned with an internal focus within an electrolyte volume disposed in a parabolic reflector, thus producing sound waves which can be focused on points outside the reflector.
It is accordingly an object of the invention to provide an apparatus for producing shock waves for technical, preferably medical applications, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type, which operates by a thermohydraulic method and through the use of which several million pulses can be generated without problems of wear.
With the foregoing and other objects in view there is provided, in accordance with the invention, an apparatus for producing shock waves for technical, preferably medical, applications, in particular for lithotripsy or pain therapy, comprising a conductive liquid electrolyte; two electrodes enclosing the conductive liquid electrolyte; and a power pulse generator controlling the electrodes with an intense electric pulse for converting electrical energy directly and very largely without losses to briefly heat the conductive liquid electrolyte and produce pressure pulsations output as acoustic sound waves of specified wavelength and high energy density into a sound propagation medium.
The invention starts from the fact that a highly conductive electrolyte is heated up briefly through the use of an intense electric pulse and the electric energy being input is converted directly and very largely without losses into thermal energy of the electrolyte. The heat can be applied simultaneously and homogeneously to relatively large, scaleable volumes and large, likewise scaleable surfaces. When heating up a large-area layer of liquid by direct current flow, the current density and electric field strength within the layer of liquid remain largely constant, with the thickness of the layer of liquid being less than the wavelength to be produced but the transverse dimension being large in comparison. In a suitable medium, the thermal expansion of the heated electrolyte produces a rise in pressure and therefore, given suitable boundary conditions, it produces a pressure wave which can propagate in this medium.
Almost any desired scaleability and geometry in combination with virtually wear-free performance of such a thermohydraulic shock wave transducer is possible due to the principle according to the invention. In contrast to the electrohydraulic principle, there is generally no concentration in the current flow due to plasma formation at individual points on the electrodes. Therefore, the operation of such a configuration does not lead to erosion of the electrodes, thereby making it possible to achieve a long service life. Due to the spatially homogeneous power loading of the electrolyte, the membrane or acoustically xe2x80x9cpermeablexe2x80x9d electrode is subject to very homogeneous mechanical loading, thereby likewise greatly increasing the service life of the membrane in comparison with electromagnetic sound transducers.
Overall, configurations in accordance with the present invention have the advantage of permitting large volumes to be subjected to uniform and homogeneous loads over a large area up to the limit of breakdown strength. This is due to the deliberate avoidance of structures which intensify the field, such as wires, peaks, edges or constrictions in the current-bearing area. Thus there is no limitation with regard to pulse energy and scaleability. The advantage of the new configuration is, in particular, that the wave fronts which arise are very uniform, with the result that a pulsed sound source that can be scaled in a virtually unlimited manner is provided and in addition a high-quality wave front is obtained.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an apparatus for producing shock waves for technical, preferably medical applications, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.